docs: def fmt_ac_ria(ria):
      """Format an AcRepertoireIrreducibilityAnalysis."""
      causality = {
          Direction.CAUSE: (fmt_mechanism(ria.purview, ria.node_labels),
                            ARROW_LEFT,
                            fmt_mechanism(ria.mechanism, ria.node_labels)),
          Direction.EFFECT: (fmt_mechanism(ria.mechanism, ria.node_labels),
                             ARROW_RIGHT,
                             fmt_mechanism(ria.purview, ria.node_labels))
      }[ria.direction]
      causality = ' '.join(causality)
  
      return '{ALPHA} = {alpha}  {causality}'.format(
          ALPHA=ALPHA,
          alpha=round(ria.alpha, 4),
          causality=causality)